1. Field of the Invention
The invention relates generally to radio antennas and more specifically to combinations of antennas suited for use with global positioning system receivers equipped with differential-correction beacon receivers.
2. Description of the Prior Art
Global positioning system (GPS) receivers can either be one of two types, authorized or unauthorized. The authorized GPS receivers are able to receive and decode a second carrier channel (L2) from the orbiting GPS satellites that carries precision code (P-code) data that must be decrypted with a special military decryption device. When selective availability (SA) is engaged by the government, the position accuracy of unauthorized GPS receivers is degraded because such receivers are able to only use the coarse acquisition (C/A) code available on the primary carrier channel (L1), and that data is deliberately dithered during SA. Position solutions that are computed therefore become randomly skewed over time in heading and distance from the perfect solution.
Since all stations in an area will be more-or-less equally affected by SA, stations with known fixed locations can assess the dither offsets by comparing GPS computed positions with the known position. Such differential correction signals can then be broadcast in real-time on a low frequency beacon channel to be used by GPS receivers in the area to correct their computed positions by an appropriate direction and magnitude. Differential GPS can provide two to five meter accuracy for even unauthorized GPS receivers. Such a beacon station is in operation at Montauk, N.Y.
Commercial GPS receivers have evolved to the point that special input/output (I/O) ports are provided on them to accept differential correction data from a separate beacon receiver. For example, Trimble Navigation (Sunnyvale, Calif.) provides a 4000 RL REFERENCE LOCATOR.TM. device that can calculate and transmit differential corrections to mobile GPS receivers. It can be configured with either eight or twelve channels to track all the GPS satellites in view. A common differential correction data format used in the industry is called "RTCM-SC104". Many commercial products are equipped to generate and receive RTCM-SC104 data.
Combined GPS and differential beacon receivers are desirable because separate components can be awkward and unwieldy in mobile use, e.g., in small boats or infantry units in the field. A combination of antennas is therefore required, but the respective GPS antennas and beacon antennas have special requirements for shielding and access to an unobstructed sky.
GPS antennas operate at such high frequencies and at such low signal levels that a typical patch or folded dipole antenna cannot be shadowed or covered by the receiver's enclosure or other circuitry. GPS antennas are therefore typically mounted upright and atop the unit with nothing more than a small plastic radome to keep out the weather and to provide mechanical protection.
Beacon receiver magnetic loop antennas, for example those operating at 300 kilohertz (KHz), need Faraday shielding to block out the electrostatic field and keep the received background noise to a minimum. The prior art includes the use of ferrite rod magnetic field antennas with Faraday shields constructed of ribbon cable circumferencially wound in an orbit around each ferrite rod and cut and soldered at one end to form a grounded comb cylinder. Such construction is labor intensive and can lower the antenna-Q of the magnetic field antenna.